The present invention relates to load lifting devices, especially such devices wherein a boom pivots in order to lift or reposition a load. An example of such a device is an aerial work platform.
A machine which lifts a load typically experiences forces which tend to tip the machine in the direction of the load as the load is lifted. This occurs, for example, in cranes and aerial work platforms. Conventionally, a counterweight may be provided for the machine in a position which tends to tip the machine in an opposite direction. This provides for better balance during operation, and less stress on certain portions of the apparatus.
In cranes, it is conventional to lift a load by means of a cable, taking up the cable in order to lift a load. It is known to provide a counterweight which shifts in a direction away from the load as the load is lifted in order to balance the crane during the lifting operation. Typically, a crane serves the purpose of lifting and moving a load from one place to another.
A aerial work platform is an example of a machine which lifts a load by pivoting a boom which supports the load. Also, it is typical that the load is supported in varying positions for extended periods of time, rather than merely being lifted from one place and set down in another. The boom may be pivoted in a generally upward direction to lift the load, an in an opposite direction to lower the load or position the load at a lesser height.
In such an apparatus, wherein the load supported by the boom may be considered to be positioned to what will be considered the xe2x80x9cfrontxe2x80x9d of the apparatus, the load tends to tip the machine forwardly. A counterweight may typically be positioned at an opposite side of the apparatus at a position which would tend to tip the machine rearwardly. This tends to roughly balance the machine.
However, as the load is lifted by the pivoting boom, the mass of the load and the boom moves in a rearward direction, closer to the balance point of the machine. As a result, the force which tends to tip the machine in a forward direction is reduced progressively as the load is lifted. If the counterweight remains stationary, the net force which may tend to tip the machine rearwardly will increase. If the change in balance is of substantial magnitude, an imbalance situation may occur.
It is known to be desirable to shift the counterweight as the load is lifted in order to better balance the apparatus. It is also desirable to lower the center the gravity of the counterweight as the load is lifted in order to maintain a lower overall center gravity of the apparatus, thus further enhancing stability.
Accordingly, an object of the present invention is to provide a mounting arrangement for a counterweight in a load lifting apparatus which permits the counterweight to shift in a manner to optimally balance the apparatus as a load is lifted or lowered. A particular object is to achieve this result in a highly efficient manner.
A further object of the invention is to provide such an apparatus wherein the counterweight is movable over a range of motion sufficient to optimally balance the apparatus.
A specific object of the invention is to provide an aerial work platform which remains optimally balanced regardless of the position of the platform, and regardless of the direction to which the platform has been rotated with respect to the supporting chassis of the aerial work platform apparatus.
The foregoing objects are achieved, in accordance with the present invention, by providing a load lifting apparatus comprising a boom which is pivotable about a horizontal axis for lifting a load, a first end of the boom extending to one side of the horizontal axis being adapted to lift a load, the boom comprising a second end extending from the horizontal axis, a linearly shiftable counterweight, and a link connecting the second end of the boom to the counterweight which shifts the counterweight in a first direction when the boom is pivoted to lift a load and in a second direction when the boom is pivoted to lower the load. The counterweight may also be shifted upwardly and downwardly in response to movement of the boom and load. In preferred embodiments, the boom and counterweight are mounted on a portion of the apparatus which is rotatable about a vertical axis.
An apparatus in accordance with the invention may alternatively comprise a hydraulically driven device, a pneumatically driven device, or an electrically driven device for shifting the counterweight in response to raising and lowering of the boom.